A device of the type stated in the introduction is shown and described in Swedish patent SE-B-342271, which relates to a device for bleaching cellulose pulp. The withdrawal of the bleaching liquid is here carried out through boxes which are disposed on the outer side of the container and into which there extend the screen arms provided, at their outer ends, with withdrawal holes. Recesses in the wall of the container are covered by plates disposed on the boxes. This construction of the diffuser gives rise to heavy leakage from the container. Moreover, the raising and lowering device for the screen pack is disposed on the bottom side of the arm of the screen pack and is constituted by a piston and cylinder assembly, the piston rod of which is fixed to a pull rod which is fastened via a ball joint to the screen arm. The screen pack is controlled in the upward and downward movement via a bush around the withdrawal pipe or by a separate control system. There is herein a strong risk of the entire screen pack becoming crooked, resulting in the pull rod being bent with the stuffing box as the breaking point, whereby the control bush is exposed to bending forces. The damage which can arise if the screen pack should tilt increases the wear on the equipment, leading rapidly to operating breakdowns.
SE-B-340 216 has previously disclosed a cellulose-bleaching tower having an axially movable screen pack, in which devices in the form of hydraulic cylinders for raising and lowering the screen packs are disposed above the screen arms. In the case of this previously known bleaching tower, the piston rod of the hydraulic cylinder is guided through a bush, as a result of which leaking hydraulic liquid can trickle down in the bush. Withdrawal is further carried out by means of boxes disposed on the contacting surface of the container as is also known from SE-B-342 271.